main.js

const form = document.getElementById("form");
const firstName = document.getElementById("firstName");
const lastName = document.getElementById("lastName");
const email = document.getElementById("email");
const password = document.getElementById("password");
const cPassword = document.getElementById("cPassword");

form.addEventListener("click", (e) => {
  e.preventDefault();

  checkInputs();
});

function checkInputs() {
  // trim to remove the whitespaces
  const firstNameValue = firstName.value.trim();
  const lastNameValue = lastName.value.trim();
  const emailValue = email.value.trim();
  const passwordValue = password.value.trim();
  const cPasswordValue = cPassword.value.trim();

  if (firstNameValue === "") {
    setErrorFor(firstName, "First Name cannot be blank");
  } else {
    setSuccessFor(firstName);
  }
  
  if (lastNameValue === "") {
    setErrorFor(lastName, "First Name cannot be blank");
  } else {
    setSuccessFor(lastName);
  }

  if (emailValue === "") {
    setErrorFor(email, "Email cannot be blank");
  } else if (!isEmail(emailValue)) {
    setErrorFor(email, "Not a valid email");
  } else {
    setSuccessFor(email);
  }

  if (passwordValue === "") {
    setErrorFor(password, "Password cannot be blank");
  } else {
    setSuccessFor(password);
  }

  if (cPasswordValue === "") {
    setErrorFor(cPassword, "Password2 cannot be blank");
  } else if (passwordValue !== cPasswordValue) {
    setErrorFor(cPassword, "Passwords does not match");
  } else {
    setSuccessFor(cPassword);
  }
}

function setErrorFor(input, message) {
  const formControl = input.parentElement;
  const small = formControl.querySelector("small");
  formControl.className = "form-control error";
  small.innerText = message;
}

function setSuccessFor(input) {
  const formControl = input.parentElement;
  formControl.className = "form-control success";
}

function isEmail(email) {
  return /^(([^<>()\[\]\\.,;:\s@"]+(\.[^<>()\[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/.test(
    email
  );
}